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Jan Koum’s Facebook post announcing his departure from the social network was far from the typical Silicon Valley farewell.

While tech executive exits are often explained under vague “personal reasons”, WhatsApp’s pistonhead founder was more up front, saying he wanted more time to collect Porsches. But what was more interesting was what he didn’t say.

Mr Koum, who was made a multi-billionaire four years ago by Mark Zuckerberg, made no mention of the Facebook founder. It almost seems ungrateful. But then Mr Koum, who sold WhatsApp to Facebook for $19bn (£14bn), never quite fit in the Bay Area mould.

The 42-year-old’s upbringing in Soviet Ukraine, before moving to California at 16, fostered a...